Amplifying system and process



R. J. ROCKWELL AMPLIFYING SYSTEM AND PROCESS Filed July 18, 1939 2Sheets-Sheet l March 4, HE.

R. J. ROCKWELL AMPLIFYING SYSTEM AND PROCESS Filed July 18, 1939 2Sheets-Sheet 2 Q i l I I I I I I I Patented Mar. 4, 1941 AMILIFYINGSYSTEM AND PROCESS Ronald James Rockwell, Cincinnati, Ohio, as-

signor to The Crosley Corporation, Cincinnati, Ohio, a corporation ofOhio Application July 18, 1939, Serial No. 285,180

15 Claims.

-This invention relates to an amplifier system, and process ofamplification, and more particularly to an amplifying system in whichone amplifying tube is capable of performing work heretofore requiringtwo tubes of the same type. The invention is particularly applicable tohigh powered class B amplifying systems, but may be employed for otherpurposes.

In many modern high powered transmitting stations, class B audiofrequency amplifier tubes are employed as plate circuit modulators forthe output tubes of the radio frequency amplifying system. For example,in a 500 k. w. transmitter,

eight 100 k. w. audio amplifier tubes operating as class B push-pullamplifiers may be employed as modulators for twelve 100 k. w. radiofrequency amplifier tubes. Each 100 k. w. tube costs in the neighborhoodof $1650 and furthermore requires approximately 7.5 k. w. tube forfilament excitation.

In accordance with the present invention, eight 100 k. w. audioamplifier tubes may be re placed by four similar tubes, not only savingthe initial cost of the tubes but providing a material saving in powerrequirements.

Such modulator tubes are ordinarily operated as class B amplifiers withone-half of the tubes in push-pull with the other half. Only one tube ofa pair is utilized at a time, as each tube operates during one-halfcycle. That is to say, the grid of one tube is driven in a positivedirection while the other tube is driven in a negative direction andsince the tubes are operated with M a negative bias substantially equalto cut off, the plate current in one tube is zero during each halfcycle. The load, therefore, shifts from one tube to the other, dependingupon which grid is driven in a positive direction.

In accordance with the present invention, the alternate half waveimpulses of the voltage or signal usually applied to the grid of one ofa pair of amplifier tubes are transferred or folded over to the otherside of the axis of the wave so that both half cycles are on the sameside of the z-xis and are applied to the grid of the same amplifiertube. After amplification, the alternate half cycles are then restoredto the opposite side of the axis so that the resulting wave form is thesame as that produced from two amplifier tubes operating in push-pull.That is, the same amplifier tube is employed for both halves of thewave. The cost and size of amplifier tubes operating as class Bamplifiers are primarily determined by the peak emission con- -.,5ditions rather than heat radiating conditions.

That is to say, the capacity of the tube is determined by the maximumusable emission of electrons from the filament rather than the powerlosses in the tube. The maximum electron emission in a given tube is notincreased 1 when one tube replaces a pair of tubes in accordance withthe present invention and one tube can perform the work ordinarilyrequiring two tubes.

It is therefore an object of the present invention to provide anamplifying system and process in which both halves of the wave to beamplified are applied to the grid of a single amplifier tube so that thetube does the work of two tubes operating in push-pull.

Another object of the invention is to provide a system and method ofamplifying in which successive half cycles of the Wave to be amplifiedare transferred to the same side of the axis so that both half wavesappear on the same side of the axis during amplification.

Another object of the invention is to provide a system of amplificationin which the wave to be amplified is rectified and then applied to thegrid of a class B amplifier tube so that one tube performs the work oftwo tubes.

A further object of the invention is to provide a system and process ofamplification in which the half cycles of the wave to be amplified arepositioned upon the same side of the axis, the resulting wave amplified,and alternate half cycles restored to the opposite side of the axis.

Other objects and advantages of the invention will appear in thefollowing description of preferred embodiments thereof shown in theattached drawings, of which:

Figure l is a schematic diagram of a circuit in accordance with thepresent invention;

Figure 2 is a diagram illustrating the operation of the conventionalclass B amplifier;

Figure 3 is a similar view illustrating the operation of an amplifier inaccordance with the present invention;

Figure 4 is a view similar to Figure 1 showing a modified amplifyingsystem; and

Figure 5 is another view similar to Figure 1, showing a further modifiedamplifying system.

Referring to Figure 2, this figure is a conventional representation ofthe operation of a class B push-pull amplifier. The curve Ill representsthe dynamic grid voltage plate current characteristic of one of thetubes and the curve ll represents the similar characteristic of theother tube. The tubes are operated at a negative bias substantiallyequal to out off so that alternate half cycles a and c of the impressedvoltage represented by the curve I2 causes current to flow in one tubeand the other alternate half cycles b and d cause current to fiow in theother tube. The curves II] and II together produce a substantiallystraight line characteristic provided the tubes are operated below theknee I3 of the curves l0 and II representing the maximum usable electronemission of the tubes. That is to say, the capacity of the tube isdetermined by the peak emission characteristics of the tube. The curveI4 represents the current in the output circuit of a conventional classB push-pull amplifier.

As illustrated in FigureB, the impressed voltage represented by thecurve I2 is in accordance with the present invent on first rectified, orsub- 'ected to a frequency doubling operation, so that the half cyclesof the signal are all on the same ide of the axis as illustrated by thecurve I5. This signal is then applied to a singleamplifier tub operatedwith a negative bias substantially equal to cut off, the dynamiccharacteristic of ,which is represented bythe curve I0. The platecurrent is represented by the curve l6 and the positions correspondingto alternate half cycles of'the original signal are then restored to theopposite side of the axis to produce a signal represented by the curveI4 which is the same as the curve: I4 of Figure 2. i

In the circuit shown in Figure l, the voltage represented by the curveI2 of Figures 2 and 3 is obtained from a center tapped secondary l6 ofan input transformer I! provided with a primary I8 connected to a sourceof voltage (not shown). Rectifying tubes I 9 and have their plates 2|connected to opposite terminals of the secondary I6 and their cathodes22 connected together and to the center tap 22? of the secondary I6through a resistor 23. It will be apparent that the tubes I9 and 20 areconnected .to form a full wave rectifier circuit and that the voltageacross resistor 23 is of the form represented by the curve I5 of Figure3. Thi voltage is impressed upon the grid 24 of a main amplifier tube25, preferably by connecting the grid 24 directly to the cathodes 22 ofthe tubes I 9 and 20. 1

The sources of the filament, plate and bias potentials are not shown inFigure 1 to avoid complicating the diagram. Bias potential is furnishedto the grid 24 of .the tube 25 from a source.

of grid potential (not shown) through the center tap 22 of thetransformer secondary I6 and the grid circuit is completed to groundthrough the source of bias potential. The plate circuit for the tube 25is completed through grid controlled rectifier tubes 21 and '28; thecenter tap 29 of the primary 30 of an output transformer 31 connectedacross the plates 3| of the tubes 21 and 28; and the source of platepotential (not shown) having its negative terminal connected to groundand its positive terminal connected to thecenter tap 29. The plate 33 ofthe amplifier tube 25 is connected to the cathodes 34 of the gridcontrolled rectifier or restoring tubes 21 and 28 and the plate currentof the tube 25 has a wave form the plate currentvrepresented by thecurve I6 is zero at the end of .eachhalf cycle. Voltages obtained fromthe circuit prior to the'rectifier tubes l9 and 20 can be impressed uponthe grids 3-5 of the tubes 2! and. 28 so that current flows in each tubeduring alternate half cycles. This voltage may be obtained from theterminals of the secondary I6 of the input transformer I1 and may beapplied to the grids 35 through a transformer 36 having a primary 3'!connected across the secondary I8 of the transformer I1 and a secondary31' having its opposite terminals connected to the grids 35 of the tubes21 and 28 and a center tap connected to the cathodes 34 of the tubes 21and 28. It will be appreciated that the secondary 31 of the transformer33 may be a tertiary winding upon the transformer I1 is sufliciently'insulated from the other windings of the transformer IT. The grid 35 ofone tube 21 will be positive, while the grid 35 of the tube 28 isnegative, and by employing tubes, preferably of the gas-filled type inwhich a negative grid prevents current flow from starting when voltageis applied to the tube, alternate half cycles may be caused to flowalternately through the tubes 21 and 28, such that the wave form of thecurrent in the secondary 35' of the transformer 3| is of the characterrepresented by the curve I4 of Figures 2 and 3.

The rectifier tubes I9 and 20, as well as restoring tubes 21 and 28, maybe of relatively small size as compared to the amplifier tube 25, sothat the cost of an initial installation is lessened and less powerrequired to produce a given amount of amplification. Thus, the currentflowing through the, tubes I9 and 20 may be small as compared to theplatecurrent of the tube 25, as the function of the tubes I9 and 20 isto rectify the voltage applied to the grid 24 of the amplifier tube 25.These tubes need only be of sufiicient size that sufficient current iscarried thereby to produce a drop across the ,resistor 23 to excite thegrid 24 and still enable a low enough value of resistance in theresistor23 to be used to enable grid current to flow therethrough if the grid 24of the tube 25 swings positive with respect to the cathode. It isentirely possible to employ gas-filled tubes for the rectifier tubes I9and 20 in order that comparatively large amounts of power may be handledby relatively small tubes. Also, the tubes 21 and 28 are preferablygas-filled tubes which can handle relatively high currents with lowvoltage drop thereacross. If the amplifier tube 25 is operated at anegative bias equal to cut on", the plate current thereof drops to zeroat the end of each half cycle of the curve l6 of Figure 3, so that thearc in one of the gas-filled tubes is extinguished and the grid thereofis capable of preventing striking of the arm when a negative potentialis placed thereon from transformer secondary 31'. Thus, four relativelysmall tubes representing a very small fraction of the initial cost of alarge amplifying tube and requiring materially less power for operationcan be em- I tially similar to the amplifying system of Figure 1; andthe same reference charactershave been applied to similar elements.Instead of employing two electrode rectifier tubes for exciting the grid24 of the amplifying tube 25, amplifier tubes 38 and 39 operated asclass B amplifiers may be employed to excite the grid 24. A source ofbias potential (not shown) for the tubes 38 and 39 may be connectedbetween the center tap 22 of the secondary I6 of the transformer l1 andthe cathodes 43 so as to bias the tubes 38 and 39 substantially to cutoff. The grids 4| of these tube may be connected to opposite terminalsof the secondary l6 of the transformer l1 and the plates 42 may beconnected together and to the positive terminal of a plate potentialsource (not shown) having its negative terminal connected to ground. Thecathodes 43 of the tube 38 and 39 may be connected to ground through aload resistor 44. If the tubes 38 and 39 are operated class B, thevoltage appearing across the resistor 44 is similar to that representedby the curve l of Figure 3. This voltage may be applied .to the grid 24of the amplifying tube 25 through a coupling condenser 45, and gridpotential can be applied to the grid 24 through a resistor 48.Sufiicient negative grid potential should be applied to the grid 24 tobring the plate current of the tube 25 to zero value at the end of eachhalf cycle. The plate current of the tube 25 will then be similar to thecurve l6 of Figure 3. The remainder of the circuit for restoringalternate half cycles of the opposite side of the axis to produce acurrent wave form similar to the curve l4 of Figure 3 may be entirelysimilar to that of Figure 1, and need not be further described.

The tubes 38 and 39 for exciting the grid of the amplifying tube 25 areessentially voltage amplifiers and can carry a relatively low platecurrent as compared .to the tube 25 and may therefore be of relativelysmall size. The particular coupling arrangement between the tubes 38 and39 and the grid 24 of the tube 25 has the disadvantage that the input tothe tube 25 is of relatively high impedance, so that the grid 24 of thetube 25 must not be driven to any considerable extent positive ifdistortion is to be minimized in the amplifying system.

As shown in Figure 5, it is possible to directly connect the grid 24 ofthe amplifier tube 25 to the cathodes of the tubes 38 and 39 so that theinput to the grid 24 of the tube 25 can be made of relatively lowimpedance and the grid 24 can be driven positive. A source of biaspotential 48 is shown as being connected between the center tap 22' ofthe transformer secondary l6 and the cathodes 43 of the tubes 38 and 39so as to bias the tubes 38 and 39 substantially to cut off. The cathodes43 are also connected to ground through a resistor 49 and a source 50 ofbias potential for the grid 24 of the tube 25. Plates 42 of the tubes 38and 39 may be connected to an intermediate point 5| of a source 52 ofplate potential in order to secure proper plate voltage for the tubes 38and 39. The cathode current for the tubes 38 and 39 flows through theresistor 49 and the potential source 50 so that the source 58 must be ofsufficiently low impedance to carry this cathode current. The voltageapplied to the grid 2 of the tube 25 is of the wave form represented bythe curve l6 of Figure 3 superimposed upon a direct current voltage,which latter voltage is just suflicient to render the plate current ofthe tube 25 zero except when the grid 24 is driven in a positivedirection. By employing tubes 38 and 39 capable of developing aconsiderable amount of power, the resistor 49 may be of sufiiciently lowimpedance that the grid 24 in the tube 25 may be driven substantiallypositive without causing major distortion in the amplifying system.

The remainder of the circuit for restoring alternate half cycles of theplate current of the tube 25'to the opposite side of the axis may beessentially similar to that shown in Figures 1 and 4 and employ gridcontrolled rectifying tubes 21 and 28. In many instances it may bedesirable to apply a slight negative bias potential to the .grids 35 ofthe tubes 21 and 28 in order to insure that these grids are slightlynegative at the start of each half cycle so that only the tube havingits grid driven in a positive direction will permit current flowtherethrough. This may be accomplished by connecting an insulated source53 of bias potential between cathodes 34 of the tubes 21 and 28 and thecenter tap of the secondary 31' of the transformer 36. It is alsodesirable to connect a resistor between the cathodes and plates of therectifier tubes 21 and 28 in order to reduce the peak inverse voltagethereacross. This may be accomplished by connecting a resistor 54 to thecenter tap 29 of the primary 30 of the transformer 3| and to thecathodes 34 through the potential source 53. This connection alsoprovides for maintaining plates of the tubes 21 and 28 at a slightnegative potential with respect to the cathodes at the end of each halfcycle so as to insure extinguishing the arc in the tube operating on theprevious half cycle so that the load can be transferred to the othertube. The sources of bias and plate potential have been shown asbatteries, but any other suitable sources of direct current may beemployed.

The invention has particular utility in connection with class Bamplifying systems, but it is evident that it is also directlyapplicable to any amplifying system in which the plate current of theamplifier tube is zero at the end of each half cycle, for example classC amplifying systems. Also, by applying a proper bias to the restoringtubes 21 and 28 in a manner similar to that shown in Figure 5 so thatthe plates or grids of the tubes2l and 28 are at zero potential or evenslightly negative with respect to the cathodes at the end of each halfcycle, the amplifier tube 25 may be operated class AB or even class Aand the wave form restored so that successive half cycles are onopposite sides of the axis as illustrated by the curves M of Figures 2and 3.

While I have disclosed the preferred embodiments of my invention, it isunderstood that the details thereof may be varied within the scope ofthe following claims.

I claim:

1. A system for amplifying an alternating current wave, which comprises,means for transferring alternate half cycles of said wave to the otherside of the axis of the Wave so that all of the half cycles arepositioned on the same side of said axis, means for amplifying theresulting wave, and means for restoring the amplified wave to a formsimilar to the original wave.

2. A system for amplifying an alternating current wave, which comprises,means for transferring alternate half cycles of said wave to the otherside of the axis of the wave so that all of the half cycles arepositioned on the same side of said axis, means for amplifying theresulting wave, and means. for restoring the portions of the amplifiedwave corresponding to alternate half cycles of the original wave to theopposite side of the axis .to produce an amplified wave having a formsimilar to the original wave.

3. A system for amplifying an alternating current wave, which comprises,full wave rectifying means for rectifying said wave so that the halfcycles of the original wave are positioned on the same side of the axisof the wave, an. amplifying tube including a grid, plateand cathode,means for impressing said rectified wave on the grid of saidamplifying'tube to produce a plate current through said tube'having awaveform similar to said rectifiedwave, and means for inverting plifyingthe resulting Wave, a pair of grid controlledtubes each connected tosaid amplifying means in series therewith, and means for applyingvoltages to the grids of said tubes to provide for current flowalternately through said tubes during time intervals corresponding tohalf cycles of the original wave, said tubes being connected to combinesaid current flow so that a current having a' final wave shape similarto that of the original wave is produced.

- 5.' A system for amplifying an alternating current wave, whichcomprises full wave rectifying means to rectifysaid wave, amplifyingmeans for amplifying said rectified wave, a pair of grid controlledtubes'each connected to said amplifying means-in series therewith, meansfor applying voltages to the grids of said tubes to provide for the flowof current from said amplifying means alternately through saidtubesduring time intervals corresponding to half cycles of said originalwa've, an output transformer provided with a secondary-winding and aprimary winding having a" center tap, a source of potential connectedbetween'said amplifying means-and said center tap, the plates of saidtubes being connected to opposite-terminals of said primary winding sothat said current flows alternatelyin opposite directions in saidprimary winding -to produce an output current in said secondary windinghaving a wave shape similar to'that of the original wave.

' 6. A system for amplifying an alternating current" wave, whichcomprises,'full wave rectifying means for rectifying said wave, anamplifier tube having a grid, plate and cathode, and connected tooperate as-a class B amplifier, means for applying said rectified waveto the grid of said tube whereby the plate current of said tube is zeroat the end of each half cycle of the original wave, a pair of gas filledgrid controlled tubes each provided with a cathode and plate and eachhaving its cathode and plate connected in series with the plate andcathode of saidamplifier tube, means for applying a voltage having awave shape similar to the original wave to the grid of one of said gasfilled tubes and a similar voltage in phase opposition to the grid ofthe other gas filled tube whereby current flows through one of said gasfilled tubes forv alternate half cycles of the original wave and throughthe other gas filled tube for the other half cycles of the originalwave, and means for combining the currents through said gas filled tubesto produce a wave form similar to said original wave.

7. A system for amplifying an audio frequency signal, which comprises, apair .of relatively small amplifier tubes arranged in a circuit forpush-pull operation and having a common cathode circuit with animpedance therein, said tubes havinga grid bias so that substantially-nocurrent flows therethrough whenno signal is impressed thereon wherebya'fvoltage is'produc'ed across saidim-.

pedance in which all half cycles of the signal are on the same side ofthe axis of the original signal, a relatively large amplifier tube,means for impressing said voltage upon the grid of said relatively largetube, said relatively large tube having a grid bias so thatsubstantially no current flows therethrough when no voltage is appliedto the grid thereof, a pair of relatively small grid controlledgas-filled .restoring tubes each connected to' said relatively largeamplifier -tube in series therewith, said restoring tubes being of thetype in which a negative grid prevents starting of current flow and apositive grid permits current fiow, means for applying voltage similarto the original signal but in phase opposition to the grids of saidrestoring tubes, and means for combining the currents through saidrestoring tubes to produce an amplified signal having a wave shapesimilar to the wave shape of the original signal.

8. A system for amplifying a signal, which comprises, a pair ofrelatively small rectifier tubes connected for full wave rectificationof said signal, a relatively large amplifier tube, means for impressingthe rectified signal upon said amplifier tube to produce an amplifiedrectifiedsignal, and means including a pair of relatively small gridcontrolled tubes forrestoring said amplified rectified signal tosubstantially the Wave shape of the original signal.

9. A system for amplifying an alternating current signal, whichcomprises, means including a pair of relatively small tubes forproducing full wave rectification of said signal, means including arelatively large amplifier tube for amplifying said rectified signal,and means including a pair of relatively small grid controlled tubes forrestoring said amplified rectified signal to sub stantially the waveshape of the original signal;

10. A system for amplifying an alternating current signal, whichcomprises, means including a pair of relatively small tubes forproducing full wave rectification of said signal, means including arelatively large amplifier-tube foramplify ing said rectified signal,and means including a pair of relatively small grid controlled tubes andconnections for impressing voltages derived from said signal prior torectification upon the grids of said last mentioned tubes for restoringsaid amplified rectified signal to substantially the wave shape of theoriginal signal. 11. A system for amplifying an alternating currentsignal, which comprises, means including a pair of tubes for producingfull wave rectification of said signal, means including an amplifiertube for amplifying said rectified signal, and means including a pair ofgrid controlled tubes and connections for impressing voltages derivedfrom said signal prior to rectification upon the grids of said lastmentioned tubes for restoring said amplified rectified signal tosubstantially the wave shape of the original signal. 12. The methodo'famplifying an alternating current wave, which comprises, transferringalternate half cycles of said wave to the other side of the axis thereofso that all of the half cycles of the wave are positioned on the sameside of said axis, amplifying the resulting wave, and thereafterrestoring alternate portions of the amplified wave corresponding toalternate half cyclesof the original wave to the opposite side of theaxis to produce an amplified wave having a wave shape similarto theoriginal wave.

l3. The'method of lamplifying' a signal, which comprises, rectifyingsaid signal to produce a'full 15. A system foramplifying an alternatingcurrent signal, which comprises, means for transferring alternate halfcycles of said signal to the other side of the axis of the signal Waveso that all of the half cycles are positioned on the same side of saidaxis, means for amplifying the resulting signal, and means responsive tovoltages derived from the original signal and having substantially thesame wave form as said original signal for restoring the amplifiedsignal to a form 10 similar to the original signal.

RONALD JAB/[ES ROCKWELL.

